Some people sell 4*s for luna but very rarely. Even more rarely people sell their TE/pot for anywhere from 5-10 million luna but it's not worth giving up all your luna for 1 te unless you have tens of millions of it banked.

A tradeable currency luna is the lowest on the picking list. As of current market prices you can commonly buy or sell :

Feeder Units (1*2*) at a rate of 50k/500k per set (the 3*sets still go for 1TE ea)

Prior to tradelock players have begun to purchase/trading fresh 4* units at 1mill but these are few and far in between. Expect luna to generally remain useless until the market depreciates enough for us to value 4*s at a currency lower than TE. It'll happen sooner rather than later so theres hope for luna yet.

( Other notable quirks would be players buying Luna with TE/Pots. Going rate 2 weeks ago was 3million per TE. As of last week a player i know has been buying for 10million per TE. Don't count on these as income though they are still rare to find. )